Zusammenfassung

Nowadays carbohydrate solutions find application in many industrial processes, especially in biological systems and food and beverage industry. They ﬁnd application not only in the food and beverage industries, they are also the basis of other important manufacturing, e.g., textiles, paper, pharmaceuticals, and chemicals. Carbohydrates are produced as mixtures of different types of sugars; therefore, further separation is required for most of applications. In order to design a separation process, the knowledge of accurate thermodynamic and rate parameters is of great importance.
The work to be presented focuses on the adsorption equilibrium of three carbohydrates: sucrose,
D-glucose and D-fructose, which are especially important in biological systems and food industry. Experimental data of single, binary and ternary mixtures of investigated sugars were acquired by frontal analysis and adsorption-desorption method using Ca2+-form ion-exchange resin [1]. The experiments covered concentration range up to 600 g/L at 60ºC. Within this range the adsorption isotherms of carbohydrates exhibited concave shape characteristic for anti-Langmuirian behavior. Data of mixture adsorption revealed reversed competitive (synergistic or cooperative) effects, i.e., an increase of the concentration of one component of the mixture enhanced the adsorption of others.
Additionally, dynamic concentration proﬁles of multicomponent mixtures have been recorded. The shapes of adsorption and desorption curves conﬁrmed observed competitive effects found in the equilibrium studies.
Initially, to model such an adsorption behavior the anti-Langmuir model has been used [1]. The isotherm parameters determined for single components were used to simulate the competitive adsorption equilibria through the IAS (ideal adsorbed solution) theory. Theoretical predictions reproduced concave isotherm shapes with satisfactory agreement with experimental data. However, such an empirical model does not take into account the effect of non-ideal phase behavior of components in highly concentrated solutions. Since the adsorption mechanism and the origin of the isotherm curvature were still not clear, in the next step, a different approach has been employed to describe adsorption behavior of carbohydrates.
Second approach consisted in the assumption that the deviation from ideal behavior can be accommodated by modifying the concentration by the introduction of activity coefficients [2]. Activities of individual sugars in aqueous solutions were quantiﬁed on the basis of solubility properties. Solid–liquid equilibria of sugars were correlated with the NRTL (nonrandom, two liquid) model of activity coefficient formulation. Activities of individual sugars were incorporated into the single component adsorption isotherm model, which reproduced accurately the course of the adsorption equilibria obtained experimentally. Activities of sugars determined in binary solute systems along with the single component isotherms were used to predict competitive adsorption equilibria. To calculate adsorbed phase concentrations of individual sugars in binary mixtures the adsorbed solution theory was adopted. The isotherm shapes calculated were compared to the data of competitive adsorption from the former study and found to be able to describe these experimental results with better accuracy than previous approach.
[1]. J. Nowak, K. Gedicke, D. Antos, W. Piątkowski, A. Seidel-Morgenstern, J. Chromatogr. A 1164 (2007)
[2]. J. Nowak, et al., J. Chromatogr. A (2009), doi:10.1016/j.chroma.2009.01.043